I guess my biggest fear is that you might be tempted to use the ETX without proper filtering for an alignment- not only could this potentially ruin your scope- but your eyesite would be toasted in a second- As for tracking the sun- well it should work.

Sorry to revive this old thread but I am looking to do the same thing as the OP. I have an ETX-125 and a PST. My ETX is my only tracking mount I have and I would like to do some imaging through the PST. Any suggestions?

NO, NO, NO! counter balance goes on the side opposite the PST so as to take the strain off the gears. To balance the scope about the axis so that if you releace the clutch it does not rotate around, placing the heavy side on the bottom.

I have the PST and have mounted it on the ETX-125 several times using a piggyback mount (originally from Scopetronix but clones are still around). The original mount handled the PST well if balanced properly. I have now put the ETX-125 on the much more stable Celestron SE 6/8 mount and the PST tracks effortlessly on the piggyback mount. I do think that the combination of binoviewer and PST might be a bit much for the old ETX-125 mount. For visual use, the mount was fine.